1. Chapter 12:
Advanced
File Operations

2. 12.1
File Operations

3. File Operations
File: a set of data stored on a computer, often on a disk drive
Programs can read from, write to files
Used in many applications:
Word processing
Databases
Spreadsheets
Compilers

4. Using Files Requires fstream header file
use ifstream data type for input files
use ofstream data type for output files
use fstream data type for both input, output files
Can use >>, << to read from, write to a file
Can use eof member function to test for end of input file

5. fstream Object fstream object can be used for either input or output
Must specify mode on the open statement
Sample modes:
ios::in – input
ios::out – output
Can be combined on open call:
dFile.open("class.txt", ios::in | ios::out);

6. File Access Flags

7. Using Files - Example // copy 10 numbers between files
// open the files
fstream infile("input.txt", ios::in);
fstream outfile("output.txt", ios::out);
int num;
for (int i = 1; i <= 10; i++) {
infile >> num; // use the files
outfile << num; }
infile.close(); // close the files
outfile.close();

8. Default File Open Modes
ifstream:
open for input only
file cannot be written to
open fails if file does not exist
ofstream:
open for output only
file cannot be read from
file created if no file exists
file contents erased if file exists

9. More File Open Details Can use filename, flags in definition:
ifstream gradeList("grades.txt");
File stream object set to 0 (false) if open failed:
if (!gradeList) ...
Can also check fail member function to detect file open error:
if (gradeList.fail()) ...

10. File Output Formatting
12.2
File Output Formatting

11. File Output Formatting
Use the same techniques with file stream objects as with cout: showpoint, setw(x), showprecision(x), etc.
Requires iomanip to use manipulators

13. Program 12-3 (Continued)

14. Passing File Stream Objects to Functions
12.3
Passing File Stream Objects to Functions

15. Passing File Stream Objects to Functions
It is very useful to pass file stream objects to functions
Be sure to always pass file stream objects by reference

19. More Detailed Error Testing
12.4
More Detailed Error Testing

20. More Detailed Error Testing
Can examine error state bits to determine stream status
Bits tested/cleared by stream member functions
ios::eofbit
set when end of file detected
ios::failbit
set when operation failed
ios::hardfail
set when error occurred and no recovery
ios::badbit
set when invalid operation attempted
ios::goodbit
set when no other bits are set

21. Member Functions / Flags
eof()
true if eofbit set, false otherwise
fail()
true if failbit or hardfail set, false otherwise
bad()
true if badbit set, false otherwise
good()
true if goodbit set, false otherwise
clear()
clear all flags (no arguments), or clear a specific flag

22. From Program 12-6

23. Member Functions for Reading and Writing Files
12.5
Member Functions for Reading and Writing Files

24. Member Functions for Reading and Writing Files
Functions that may be used for input with whitespace, to perform single character I/O, or to return to the beginning of an input file
Member functions:
getline: reads input including whitespace
get: reads a single character
put: writes a single character

25. The getline Function Three arguments: Name of a file stream object
Name of a string object
Delimiter character of your choice
Examples, using the file stream object myFile, and the string objects name and address:
getline(myFile, name);
getline(myFile, address, '\t');
If left out, '\n' is default for third argument

28. Single Character I/O get: read a single character from a file
char letterGrade;
gradeFile.get(letterGrade);
Will read any character, including whitespace
put: write a single character to a file
reportFile.put(letterGrade);

29. Working with Multiple Files
12.6
Working with Multiple Files

30. Working with Multiple Files
Can have more than file open at a time in a program
Files may be open for input or output
Need to define file stream object for each file

34. 12.7
Binary Files

35. Binary Files Binary file contains unformatted, non-ASCII data
Indicate by using binary flag on open:
inFile.open("nums.dat", ios::in | ios::binary);

36. Binary Files Use read and write instead of <<, >> char ch;
// read in a letter from file
inFile.read(&ch, sizeof(ch));
// send a character to a file
outFile.write(&ch, sizeof(ch));
address of where to put
the data being read in.
The read function expects
to read chars
how many bytes to
read from the file

37. Binary Files
To read, write non-character data, must use a typecast operator to treat the address of the data as a character address
int num;
// read in a binary number from a file
inFile.read(reinterpret_cast<char *>&num,
sizeof(num));
// send a binary value to a file
outf.write(reinterpret_cast<char *>&num,
treat the address of num as
the address of a char

38. Creating Records with Structures
12.8
Creating Records with Structures

39. Creating Records with Structures
Can write structures to, read structures from files
To work with structures and files,
use ios::binary file flag upon open
use read, write member functions

40. Creating Records with Structures
struct TestScore {
int studentId;
double score;
char grade; };
TestScore oneTest;
...
// write out oneTest to a file
gradeFile.write(reinterpret_cast<char *> (&oneTest), sizeof(oneTest));

41. 12.9
Random-Access Files

42. Random-Access Files
Sequential access: start at beginning of file and go through data in file, in order, to end
to access 100th entry in file, go through 99 preceding entries first
Random access: access data in a file in any order
can access 100th entry directly

43. Random Access Member Functions
seekg (seek get): used with files open for input
seekp (seek put): used with files open for output
Used to go to a specific position in a file

44. Random Access Member Functions
seekg,seekp arguments:
offset: number of bytes, as a long
mode flag: starting point to compute offset
Examples:
inData.seekg(25L, ios::beg);
// set read position at 26th byte
// from beginning of file
outData.seekp(-10L, ios::cur);
// set write position 10 bytes
// before current position

45. Important Note on Random Access
If eof is true, it must be cleared before seekg or seekp:
gradeFile.clear();
gradeFile.seekg(0L, ios::beg);
// go to the beginning of the file

46. Random Access Information
tellg member function: return current byte position in input file
long int whereAmI;
whereAmI = inData.tellg();
tellp member function: return current byte position in output file
whereAmI = outData.tellp();

47. Opening a File for Both Input and Output
12.10
Opening a File for Both Input and Output

48. Opening a File for Both Input and Output
File can be open for input and output simultaneously
Supports updating a file:
read data from file into memory
update data
write data back to file
Use fstream for file object definition:
fstream gradeList("grades.dat",
ios::in | ios::out);
Can also use ios::binary flag for binary data